Add -R option to ci/build.py to avoid rebuilding containers

[larroy]

There are a couple of problems which trigger continuous rebuild of the containers which have difficult solutions, this option enables a workaround.
docker/docs#8886

@marcoabreu

In one ec2 machine:

Sending build context to Docker daemon  264.2kB                                                                                                                                     [73/1937]
Step 1/41 : FROM ubuntu:16.04
 ---> 13c9f1285025                  
Step 2/41 : WORKDIR /work/deps
 ---> Using cache 
 ---> fd89693ec7d8                              
Step 3/41 : COPY install/ubuntu_core.sh /work/
 ---> Using cache 
 ---> acc84b2e4ad2                    
Step 4/41 : RUN /work/ubuntu_core.sh
 ---> Using cache 
 ---> 97d47815b79e                              
Step 5/41 : COPY install/deb_ubuntu_ccache.sh /work/
 ---> Using cache 
 ---> 458bcdc2b9e2                    

Different machine:

build.py: 2019-07-02 01:05:05,098Z INFO Running command: 'docker build -f docker/Dockerfile.build.ubuntu_cpu --build-arg USER_ID=624416249 --build-arg GROUP_ID=600260513 --cache-from mxnetci/build.ubuntu_cpu -t mxnetci/build.ubuntu_cpu docker'
Sending build context to Docker daemon  264.2kB
Step 1/41 : FROM ubuntu:16.04
 ---> 13c9f1285025
Step 2/41 : WORKDIR /work/deps
 ---> Using cache
 ---> 44a97319ecd8
Step 3/41 : COPY install/ubuntu_core.sh /work/
 ---> d0284928a471
Step 4/41 : RUN /work/ubuntu_core.sh
 ---> Running in cc6d4d02a210
+ apt-get update
Get:1 http://archive.ubuntu.com/ubuntu xenial InRelease [247 kB]
Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [109 kB]
Get:3 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB]
Get:4 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [107 kB]
Get:5 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages [1558 kB]

I haven't found a solution for this and I think is related to different user id's and problems with docker caching.

[roywei]

@mxnet-label-bot add [CI]

[marcoabreu]

Ye it's probably user id - for whatever reason docker cares about that...

[larroy]

Danke für die merge.